the academic staff of a large university are provided with a PC each. it is known that after a number of years the PC need to be upgraded. The time interval before upgrading of the PC is distributed normally with a mean of 24 months and a standard deviation of 6 month. what is the probability that:
(a) the time interval before the PC needs upgrading is more than 2 years and 6 months.

(b) the time interval before the PC need upgrading is between 23 and 27 months.

(c) the mean time interval before the PC needs upgrading is at most 25 months out of 25 PCs chosen at random.